Fix backwards compatibility issues with feed arrays in beamfits files

[bhazelton]

Description

I discovered this problem when trying to read in a old beamfits file that @rlbyrne made for LWA data which had feeds in the feed array specified as "E" and "N", which used to be valid but are not now. This makes UVBeam backwards compatible for feeds specified that way.

Motivation and Context

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change (documentation changes only)
• Version change
• Build or continuous integration change
• Other

Checklist:

• I have read the contribution guide.
• My code follows the code style of this project.

Bug fix checklist:

• My fix includes a new test that breaks as a result of the bug (if possible).
• I have updated the CHANGELOG.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.93%. Comparing base (8db07ad) to head (ec766e4).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1638   +/-   ##
=======================================
  Coverage   99.93%   99.93%           
=======================================
  Files          67       67           
  Lines       22627    22638   +11     
=======================================
+ Hits        22613    22624   +11     
  Misses         14       14           

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